FLUENT allows you to choose the discretization scheme for the convection terms of each governing equation. (Second-order accuracy is automatically used for the viscous terms.) When the pressure-based solver is used, all equations are, by default, solved using the first-order upwind discretization for convection. When the density-based solver is used, the flow equations are solved using the second-order scheme by default, and the other equations use the first-order scheme by default. See Section 25.3 for a complete description of the discretization schemes available in FLUENT.
In addition, when you use the pressure-based solver, you can specify the pressure interpolation scheme. See Section 25.4.1 for a description of the pressure interpolation schemes available in FLUENT.